Locking Screen with LightDM

The LightDM display manager provides
dm-tool to allow us to lock the screen from the command line

$ dm-tool lock

On Ubuntu 13.10 (and others), running this command locks the screen
and displays the display manager's login screen. If we use
Ctl-Alt-F7 to get back to our desktop we will see a lock
screen login prompt. Entering our password there or on the display
manager's login screen will unlock our screen.

Issue Lubuntu 13.10 [140103] Whist the display manager
greeter is displayed, Ctl-Alt-F7 takes us back to an unlocked
desktop! This can be quite a security issue until fixed. See a
work-around below.

A similar command will also lock the screen and switch to the display
manager's login screen.